Known in the art is a procedure for preparing nitrogen-potassium fertilizer, e.g. potassium nitrate, by interaction of calcium nitrate with potassium chloride in an aqueous medium (cf. U.S. Pat. No. 3,361,522). By this procedure potassium nitrate is crystallized into a solid phase as a result of cooling the obtained solution to a temperature of -10.degree. C. to -20.degree. C. The mother solution is concentrated and double salt KNO.sub.3.CaCl.sub.2.2H.sub.2 O previously added to the initial solution is precipitated therefrom. Upon precipitating the double salt and removing calcium chloride precipitate, the mother solution is recycled as well.
The most serious disadvantages of above procedure reside in general chloride contamination of the final product, intricacy of the technological process including stages of potassium nitrate crystallization upon cooling to a temperature of -10.degree. C. to -20.degree. C., evaporation of the solutions and separation of the double salt at a temperature of 20.degree. to 25.degree. C., separation and purification of calcium chloride. The complexity of this technological process results in a great expenditure of energy.
Also known in the art are procedures (cf. U.S. Pat. No. 3,690,819, British Pat. No. 1,313,926) involving preparation of potassium nitrate by precipitating it from the solution containing calcium nitrate and potassium chloride. Upon separating potassium nitrate, the mother solution is treated with lime to produce calcium oxychloride CaCl.sub.2.3CaO.16H.sub.2 O.
The disadvantages of such procedure reside in chloride contamination of the final product, intricacy of technological operations including extra treatment of the mother solution with lime to produce calcium oxychloride. Since there is no demand for calcium oxychloride, it is to be rejected thus resulting in pollution of the environment by toxic substances.
Also known in the art is a procedure for preparation of potassium nitrate (cf. French Pat. No. 2,805,580) by interacting calcium nitrate with potassium chloride in an aqueous medium in the presence of monoalcohols, polyalcohols or ketones.
The disadvantage of above procedure is intricacy of the process at the expense of using organic compounds, which involves the necessity of their regeneration and results in substantial increase in toxicity, fire- and explosion-hazards of the process.
Also known in the art is a procedure for preparing potassium nitrate from calcium nitrate and potassium sulfate (cf. F.R.G. Pat. No. 974,061). Taken in stoichiometric ratio the starting components, i.e. potassium sulfate and calcium nitrate are introduced into the mother solution containing nearly 5% excess of calcium nitrate. The gypsum suspension thus produced is filtered, the gypsum remaining onto the filter is washed with water, the mother solution is cooled to produce potassium nitrate, while the solution is evaporated and recycled.
The above procedure suffers from the disadvantages residing in intricacy of the process due to the necessity for rigid control of the calcium nitrate content of the solution, which should be at a level of 5% in excess of the stoichiometric amount. A reduced amount of the nitrate results in substantial potassium losses through the production of syngenite, while its increased level promotes both contamination of the final product with calcium nitrate and degradation of physical-and-chemical properties of the fertilizer due to hygroscopicity of calcium nitrate.
There is also known a procedure for preparing nitrogen-potassium fertilizer by an exchange interaction of potassium sulfate with calcium nitrate in an aqueous medium followed by separation of gypsum from the potassium nitrate solution, evaporation of the mother solution and its subsequent recycling (cf. Inventor's Certificate of the USSR No. 382,595). The process of interacting calcium nitrate with potassium sulfate is carried out in an excess of the potassium sulfate which is dissolved in the circulating mother solution to produce lye of 8 to 15% concentration on the basis of K.sub.2 SO.sub.4.
The most serious disadvantage of said procedure is preparation of fertilizer having an unbalanced ratio of the nutritive elements, namely nitrogen and potassium, which results in lower agrochemical effectiveness of the desired product. By the known procedure, as a nitrogen-potassium fertilizer, potassium nitrate is prepared wherein the ratio of nitrogen to potassium oxide (K.sub.2 O) is about 1 to 3.4 (approximately 14% of nitrogen and approximately 47% of K.sub.2 O). However practical experience with fertilizers has shown the necessity for the products used as fertilizers for basic agricultural crops to be rich in nitrogen. Thus fixed nitrogen, e.g. in the form of ammonium nitrate, should be used together with potassium nitrate, which fact implies both production of the former and adoption of the equipment applied for flour mixing.
Another disadvantage of the known procedure is the probability of producing a non-homogeneous fertilizer because of stratification of its components.
One more disadvantage of the known procedure is the envisaged application of potassium sulfate in excess of stoichiometric amount. In case this condition is not met when producing potassium nitrate by the known procedure, an appreciable loss of potassium occurs through the formation of syngenite K.sub.2 Ca(SO.sub.4).sub.2.H.sub.2 O which is brought out of the process together with calcium sulfate. Thus it is essential that rigid control be exercised over the excess level of potassium sulfate, which is rather difficult under industrial conditions.